Electrostatic spray coating a cylindrical article such as, for example, a drum has been conventionally carried out by rotating drums one by one while holding them vertical, as disclosed in Japanese Patent Application Lain-Open publication No. 61672/1987.
Unfortunately, with such conventional coating, when spraying the liquid under conditions evenly forming droplets on to the drum to a degree sufficient to form a smooth film, the droplets flow down in the axial direction of the drum, resulting in the thickness of the film in the axial direction of the drum being non-constant. Also, rotation of the drums while holding them horizontal, in addition to that while holding them vertical, when electrostatic spray coating the drums one by one, results in a potential gradient which increases at the end of the drum, leading to an increase in the amount of liquid applied to the end, resulting in the thickness of the film formed on the drum being uneven. In order to avoid such a problem, it is required to arrange dummy drums on both sides of the drum to be coated. However, this substantially reduces the efficienty in coating the liquid on the drum. Furthermore, coating the drums one by one requires a drum holding and rotating mechanism for each drum, leading to not only an increase in cost but also to a failure in coating the drum symmetrically about its center in its axial direction, so causig a deterioration of the uniform spray coating on the drum. Also, this causes the drum holding and rotating mechanism to be coated likewise, resulting in a failure in the stable operation of the mechanism.
Moreover, in spray coating, it is desirable to spray liquid from a spray head continuously. Intermittent spraying of the liquid causes the liquid to dry in the spray head, leading to a failure in coating because the film formed by dried spray in the spray head is partially dissolved during a subsequent coating operation, producing foreign matter, thus, continuous coating is desired. However, conventional continuous coating leads to wastage of the liquid, resulting in an increase in cost.